1. Field of the Invention
The present invention relates to cobalt compounds for use in alkaline storage batteries, methods for manufacturing the same, and positive electrode plates of alkaline storage batteries employing the same.
2. Related Background Art
In recent years, long-life and highly reliable secondary batteries have been desired as power sources for a variety of uses, particularly for electric cars or hybrid electric vehicles. A promising example of such a secondary battery is an alkaline storage battery containing nickel hydroxide as an active material. To increase the utilization factor of the active material at a positive electrode in the foregoing alkaline storage battery, a technique of adding cobalt to the positive electrode so as to form a conductive network has been proposed. More specifically, alkaline storage batteries utilizing positive electrode plates provided by three methods as described below have been proposed.
A first method provides a positive electrode plate utilizing a nickel hydroxide active material coated with a compound containing cobalt with a valence of more than 2, and utilizing a compound containing cobalt compound with a valence of not more than 2 (see JP10(1998)-270039A).
A second method provides a positive electrode plate by filling a porous metal support with a paste containing an active material, cobalt chloride, and water (see JP10(1998)-106555A).
A third method provides a positive electrode by utilizing a paste containing CoF2 powder having an average particle size of not more than 2 μm and active material powder, the CoF2 powder being 3 wt % to 15 wt % (preferably, 8 wt % to 12 wt %) with respect to the total amount of the CoF2 powder and the active material powder (see JP7(1995)-169466A).
The foregoing conventional methods, however, have been incapable of meeting the recent demands for batteries, that is, the demands for batteries with a lifetime corresponding to that of automobiles.
Furthermore, the first method requires steps of forming a coating layer of a cobalt compound on a surface of the active material and oxidizing the same, which make the manufacturing process complex. This complexity of the manufacturing process has made the first method inappropriate for mass production.
The second and third methods have a drawback in that an abundance of chloride ions and fluorine ions remain in an electrolytic solution after formation of Co(OH)2, which hinders the reaction of the active material, thereby decreasing the utilization factor of the same. Besides, since hydroxide ions in the electrolytic solution and anions of the cobalt compound are consumed upon the formation of Co(OH)2, the Co(OH)2 forming reaction is not uniform, thereby causing the concentration of the electrolytic solution to vary. Therefore, a drawback has arisen in that battery properties tend to become non-uniform. Furthermore, Co(OH)2 is deposited on not only the electrode plate but also a separator, thereby impairing the properties in some cases.